METHOD FOR DETECTING A DETERIORATION OF A RUBBER BEARING

Abstract: The electrical impedance of a dumbbell rubber sheet (1) is measured. From the results of the measurement, the deterioration of the dumbbell rubber sheet (1) is detected.

Description: TECHNICAL FIELD The present invention relates to a method of detecting a deterioration of a rubber bearing including a rubber member, wherein the rubber bearing is disposed in a lower portion of a building to receive a load from the building. Background Art A rubber bearing which is comprised of a rubber member and a reinforcing member is used in supporting a structure such as a building and the like. As a result of the fact that such rubber bearings are generally disposed in a lower portion of the structure, the rubber bearings receive the load of the structure over a long period of time, and are also subjected to shear stress due to earthquakes and the like, while they gradually deteriorate due to aging or the like. When the deterioration of a fixed level or more has occurred, it is necessary to replace the rubber bearings, and the extent of the deterioration of the rubber member of the rubber bearing is conventionally ascertained by visual observation of a configuration or dimensional change in the overall configuration of the rubber member, a crack or an exfoliation in the rubber member or the like, or by the use of a measuring instrument such as a strain gauge, an ultrasonic test instrument, or a measure. Since the rubber bearings are generally disposed in a lower portion of the structure as described above, the visual observation of the rubber members is very difficult since an inspector must enter the lower portion of the structure and conduct the visual observation. In addition, the inspector must rely on his experience and intuition in the observation, so that there is a possibility that the results of observation may vary. Meanwhile, in the observation of the rubber members by the use of measuring instruments such as those described above, it is possible to obtain satisfactory results to a certain extent when a dimensional change is involved. When a dimensional change is not involved, it is impossible to obtain desired results. In order to monitor the supporting performance of the rubber bearings during the period of use, it is necessary to remove the rubber bearings from the structure, or separately prepare in advance a rubber bearing specimen of the same material as that of the rubber bearings installed under the structure, and to conduct a dynamic test by applying a fixed axial force to the rubber bearing specimen after the lapse of a predetermined period. Thus, it has been impossible to monitor the supporting performance of the rubber bearings in use as installed under the structure. Such a problem occurs not only with the rubber member itself of the rubber bearing, but, generally speaking, it can similarly occur with rubber members applied to various members which are formed of material such as natural rubber, synthetic rubber, or the like. JP-A-55119032 describes a method of measuring the quantity of deterioration of a rubber hose. Electroconductive rubber is inlaid in the rubber hose in its longitudinal direction and metal members in contact with the electroconductive rubber are used to measure the electric resistance. JP-A-04147024 describes the measurement of various changes in a shape of a rubber material having conductive particles dispersed inside. By measuring the internal resistance due to a strain of the rubber material, a change in the shape can be detected. EP 0312623 A1 describes a dielectric probe for measuring, monitoring and/or controlling the characteristics of a medium which may be contacted with the probe. The probe is designed for use with conventional frequency and time domain impedance analysers. The probe is a thin, flat capacitor probe which comprises an array of straight or curved electrode lines. Two arrays of lines form two terminals of a capacitor. The material to be measured is to be placed in contact with the arrays of lines and the dielectric properties of the material can be measured. The probe permits measurement of a material during curing of a resin while the material undergoes changes in its physical state. The probe can be used for measuring the dielectric properties of rubbers. The present invention has been devised in view of the above described aspects, and its object is to provide a method of detection a deterioration of a rubber bearing including a rubber member, wherein the rubber bearing is disposed in a lower portion of a building to receive a load from the building, the method being capable of reliably observing the extent of deterioration even when a dimensional change is not involved and making it unnecessary for an inspector to enter a lower portion of a.building at the time of observation, and making it possible to reliably ascertain the extent of deterioration remotely. DISCLOSURE OF INVENTION In accordance with the present invention, the above-described object is attained by a method as defined in patent claim 1. The rubber bearing may comprise a reinforcing member. Here, in a case where the reinforcing member is made of a metal, the electrical impedance of the rubber member may be measured by using the reinforcing member as an electrode. Furthermore, in a case where the rubber bearing further has flange plates respectively disposed on opposite end surfaces of the rubber member, the electrical impedance of the rubber member may be measured by using the flange plates as electrodes. The material of the rubber member, which is the object to be measured in the present invention, may be natural rubber, synthetic rubber, or a mixture thereof. In the case of the rubber bearing which is used in an earthquake-resistant device, an isolator, and a vibration-control device, there are cases where high-damping rubber is used, and the method of the present invention can be suitably applied to the rubber member made of such a high-damping rubber. The rubber bearing used in the earthquake-resistant device, the isolator, and the vibration-control device is formed with a lead plug additionally disposed in the center of the rubber member, as necessary, and the method of the present invention can be suitably applied to such a rubber bearing incorporating the lead plug. In the present invention, electrodes are disposed such that the rubber member whose change in the property is to be detected is placed therebetween. A voltage with a frequency of, for instance, 200 kHz is applied across these electrodes, and the electrical impedance of the rubber member is measured from a current value. Here, the electrical impedance changes in correspondence with the change in the property of the rubber member, e.g., the generation of the internal strain, so that the change in the property of the rubber member can be detected from the result of the measurement. As described above, in accordance with the present invention, even when a dimensional change is not involved, it is possible to reliably observe the extent of deterioration of the rubber member, and, it is unnecessary for an inspector to enter a lower portion of the structure at the time of observation, and it is possible to reliably ascertain the extent of deterioration remotely. Hereafter, a description will be given of the present invention with reference to the embodiments shown in the drawings. It should be noted that the present invention is not limited to these embodiments. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a plan view of a rubber sheet used in Example 1 for explaining the method of the present invention; Fig. 2 is a front elevational view explaining a measuring method in accordance with Example 1; Fig. 3 is the result of measurement of the relationship between the displacement of the rubber sheet and the impedance between electrodes in Example 1; Fig. 4 is the result of measurement of the relationship between a force applied to the rubber sheet and the impedance between the electrodes in Example 1; Fig. 5 is the result of measurement of the relationship between the displacement of the rubber sheet and the force applied to the rubber sheet in Example 1; Fig. 6 is the result of measurement of the relationship between the elapsed time and the impedance between the electrodes in Example 1; Fig. 7 is the result of measurement of the relationship between the elongation and the thickness in Example 1; Fig. 8 is a cross-sectional view of a rubber bearing used in Example 2 for explaining the method of the present invention; Fig. 9 is the result of measurement of the relationship between a vertical displacement occurring to the rubber member and a vertical force applied to the rubber member in Example 2; Fig. 10 is the result of measurement of the relationship between the vertical displacement occurring to the rubber member and the impedance between inner steel plates in Example 2; Fig. 11 is the result of measurement of the relationship between a vertical force applied to the rubber member and the impedance between the inner steel plates in Example 2; Fig. 12 is the result of measurement of the relationship between the elapsed time and the impedance between the electrodes in Example 2; and Fig. 13 is a cross-sectional view of another example of a rubber bearing to which the method of the present invention is applicable. EMBODIMENTS Example 1 A dumbbell-shaped rubber sheet 1 of No. 1 type of JIS (K6301) such as the one shown in Fig. 1 was prepared as an object to be measured. As for this rubber sheet 1 as a rubber member, its overall length L was 120 mm, the length 1 of its small-width portion was 40 mm, the width W of its large-width portion was 25 mm, the width w of its small-width portion was 10 mm, and its thickness t was 2 mm. Then, as shown in Fig. 2, electrodes 2 and 3, which were formed of copper plates having a diameter of 5 mm and a thickness of 1 mm, were applied to both sides of the rubber sheet 1, and they were clamped and held by polyethylene plates 4 and 5. Under the conditions of the temperature: 25°C, a deformation rate: 10 mm/min, and a pressing force, an elongating force in the direction of arrow A (in the longitudinal direction) was applied to this rubber sheet 1 repeatedly four times such that the elongation percentage (100% = 50 mm) became 0% - 25% - 0% - 50% - 0% - 75% - 0% - 100% - 0%. At the same time, a voltage with a frequency of 200 kHz was applied across the electrodes 2 and 3. The results of measurement are shown in Figs. 3 to 6. Here, Fig. 3 shows the result of measurement of the relationship between the displacement of the rubber sheet 1 and the impedance between the electrodes 2 and 3. Fig. 4 shows the result of measurement of the relationship between the force applied to the rubber sheet 1 and the impedance between the electrodes 2 and 3. Fig. 5 shows the result of measurement of the relationship between the displacement of the rubber sheet 1 and the force applied to the rubber sheet 1. Fig. 6. shows the result of measurement of the relationship between the elapsed time and the impedance between the electrodes 2 and 3. Incidentally, as the measuring instrument, the LF Impedance Analyzer (Model 4129A manufactured by Yokogawa-Hewlett-Packard, Ltd.) was used. Focusing attention on Fig. 6, it can be seen that the impedance changes in the order of a, b, c, d, e, f, g, h, and i in correspondence with the application of the elongating force corresponding to the elongating percentage of 0% - 25% - 0% - 50% - 0% - 75% - 0% 100% - 0%. Also, it can be seen that the impedances c, e, g, and i are respectively greater than the original impedance a, and that the impedance gradually increases in the order of c, e, g, and i in correspondence with an increase in the number of times the elongating force is applied. Accordingly, by detecting the change in impedance from the original impedance, it is possible to detect the application of the elongating force to the rubber sheet 1, i.e., the occurrence of the internal strain in the rubber sheet 1. In addition, by detecting the amount of increase in impedance, it is possible to detect the extent of deterioration of the rubber sheet 1 ascribable to the generation of the repeated internal strains. The large changes exhibited by the impedances b, d, f, and h are conceivably attributable to the changes in the thickness of the rubber sheet 1 due to the elongation. For reference, Fig. 7 shows the result of measurement of the relationship between elongation and thickness. Example 2 A rubber bearing 10 such as the one shown in Fig. 8 was prepared. The rubber bearing 10 is comprised of disk-shaped upper and lower connecting steel plates (upper and lower flange plates) 11 and 12, a cylindrical rubber member 13, and three disk-shaped inner steel plates 14, 15, and 16 serving as reinforcing members embedded in the rubber member 13. Lead wires are respectively screwed to the inner steel plates 14 and 15 of the rubber bearing 10, the lead wires were led to the outside, and the inner steel plates 14 and 15 were used as electrodes. Under the temperature condition of 23°C, a uniform stationary compressive force with a surface pressure of 3922660 Pa (40 kgf/cm 2 ) and a load of 43737.659 N (4.46 tonf) was applied to the rubber bearing 10 repeatedly three times in the direction of arrow B (in the vertical direction). At the same time, a voltage with a frequency of 100 kHz was applied across the inner steel plates 14 and 15. The results of measurement are shown in Figs. 9 to 12. Fig. 9 shows the result of measurement of the relationship between the vertical displacement occurring in the rubber member 13 and the vertical force applied to the rubber member 13. Fig. 10 shows the result of measurement of the relationship between the vertical displacement occurring in the rubber member 13 and the impedance between the inner steel plates 14 and 15. Fig. 11 shows the result of measurement of the relationship between the vertical force applied to the rubber member 13 and the impedance between the inner steel plates 14 and 15. Fig. 12 shows the result of measurement of the relationship between the elapsed time and the impedance between the inner steel plates 14 and 15. Incidentally, a measuring instrument similar to the one used in Example 1 was used as the measuring instrument. Focusing attention on Fig. 12, it can be seen that the impedance changes in the order of a, b, c, d, e, f, and g in correspondence with the compressive load applied repeatedly three times in the order of 0 - 43737.659 N - 0 - 43737.659 N - 0 - 43737.659 N - 0 (0 - 4.46 tonf - 0 - 4.46 tonf - 0 - 4.46 tonf - 0), and that the change in impedance is small when the compressive load is reduced. Accordingly, by detecting the change in impedance, it is possible to detect the application of a vertical force to the rubber member 13, i.e., the generation of the internal strain in the rubber member 13. In addition, by performing the integration and the like of the times the impedance changes, it is possible to detect the extent of the deterioration of the rubber member 13 ascribable to the generation of the repeated internal strain. As a result, it is possible to readily ascertain the extent of the deterioration of the rubber member 13 by introducing the lead wires to a remote place, e.g., a monitoring chamber or the like, and by monitoring the change in impedance there. It should be noted that the method of the present invention is also applicable to, in addition to the rubber bearing 10 such as the one described above, a rubber bearing 53 comprised of a rubber member 51 and an inner steel plate 52 serving as a reinforcing member embedded in the rubber member 51, as shown in Fig. 13, by, for example, attaching electrodes to one end surface 54 of the rubber member 51 and the other end surface 55 opposed thereto, respectively.

Claims: A method of detecting a deterioration of a rubber bearing (10) including a rubber member (13), wherein the rubber bearing (10) is disposed in a lower portion of a building to receive a load from the building; characterized by : measuring an electrical impedance of the rubber member (13); and detecting the change over time in impedance (c, e, g, i) at minima of repeated internal strains as compared with the original impedance (a) to detect the extent of the deterioration of the rubber member (13) ascribable to the generation of said repeated internal strains. A method according to claim 1, wherein the rubber bearing (10) comprises a reinforcing member (14, 15, 16) made of a metal, and the electrical impedance of the rubber member (13) is measured by using the reinforcing member (14, 15, 16) as an electrode. A method according to claim 1, wherein the rubber bearing (10) comprises a reinforcing member (14, 15, 16) and wherein said rubber bearing (10) further has flange plates (11, 12) respectively disposed on opposite end surfaces of said rubber member (13) and the electrical impedance of the rubber member (13) is measured by using said flange plates (11, 12) as electrodes. A method according to any one of claims 1 to 3, further comprising a step of integrating over time the change of the electrical impedance.

Inventors: AKUTSU YOUICHI, IKENAGA MASAYOSHI, OKAWA YOSHINAO, SHIMODA IKUO, AKUTSU, YOUICHI, IKENAGA, MASAYOSHI, OKAWA, YOSHINAO, SHIMODA, IKUO, AKUTSU, YOUICHI, JAPAN ATOMIC ENERGY RES. INST., OKAWA, YOSHINAO, JAPAN ATOMIC ENERGY RES. INST.
Applicants: JAPAN ATOMIC ENERGY RES INST, OILES INDUSTRY CO LTD, JAPAN ATOMIC ENERGY RESEARCH INSTITUTE, OILES CORPORATION
Application year: 1994
Main classification: G01N27
IPC codes: G01N27, G01N33